Bistable electro-optical network



United States Patent Ofiice 3,066,223 Patented Nov. 27, 1962 3,055,223BISTABLE ELECTROOPTICAL NETWORK James F. Vize, Rhinebeck, N.Y., assignorto General Electric Company, a corporation of New York Originalapplication Dec. 27, 1957, Ser. No. 705,680, now Patent No. 2,997,596,dated Aug. 22, 1961. Divided and this application Jan. 19, 1961, Ser.No. 83,730

6 Cla ms. (Cl. 250-208) This invention-relates to bistableelectro-optical networks, and more particularly to electrical networksincluding electroluminescent phosphors and photo-conductors as elementsthereof and adapted to operate in either one of two stable states.

The phenomenon of electroluminescence upon which the operation of thenetworks of the present invention in part depends is the process bywhich certain semiconducting materials, known as phosphors, emitradiation under the primary stimulus of an applied electrical field orpo tential. For a survey and bibliography on the subject ofelectroluminescence, reference is made to an article by G. Destriau andH. F. Ivey, Electroluminescence and Related Topics, Proceedings of theInstitute of Radio Engineers, vol. 43 (1955), pp. 1911-4940.

As noted in the above article electroluminescent phosphors have in thepast been used as light sources in devices frequently calledelectroluminescent capacitors or electroluminescent cells. Such devicesoften resemble a flat plate capacitor and may comprise two parallelplanar electrodes which have sandwiched between them, in one form oranother, an electroluminescent phosphor. The phosphor may be in the formof microcrystals suspended in a transparent plastic or dielectricbinder. Alternatively, the phosphor may be in the form of a continuous,transparent crystalline layer such as that disclosed in US. Patent No.2,709,765 to L. R. Koller, or in the form of single crystals asdisclosed in US. Patent No. 2,721,950 to Piper and Johnson. In generalthe microcrystal-inplastic type of phosphor dielectric exhibitselectroluminescence only under excitation by alternating electricfields, whereas in the two patents referred to above, the phosphorsexhibit electroluminescence when excited by either alternating orunidirectional electric fields. The carrierinjection electroluminescencedescribed in the aforementioned article is a type of electroluminescenceexcited by unidirectional electric fields.

Prior known electro-optical networks employing both electroluminescentphosphors and photoconductors dis posed and interconnected for mutualcooperation have been used as amplifiers or oscillators, etc. Networksof this type are shown in US. patent application Serial No. 585,027 nowPatent No. 2,904,696 by R. E. Halsted and J. F. Elliott and US. patentapplication Serial No.

585,052 by C. F. Spitzer now Patent No. 2,975,290, both applicationsbeing assigned to the assignee of the instant application.

The term photoconductor as used herein is intended to apply to anymaterial the impedance or conductivity of which varies as a function ofthe radiation emitted by a particular associated electroluminescentphosphor. A photoconductor is said to be in radiation-coupledrelationship with an electroluminescent phosphor when they are sorelated that the impedance or conductivity of the nectedin electricalseries relation and positioned in radiation-coupled relationship may betermed an electrooptical pair, for purposes of this specification. Suchan electro-optical pair, when connected across a predetermined value ofvoltage is adapted to be bistable; that is, the pair will draw only oneof two possible values of current, one high and one low.correspondingly, the intensity of radiation emitted from theelectroluminescent cell Will be high or low, depending on whether thevalue of current is high or low. When the electro-optical pair is in itsdark state (emission from the electroluminescent cell is low),application of an external radiation signal to the photoconductor willswitch the pair to its other state. If two such independently stableelectro-optical pairs are so mutually interrelated that proper signalfeedback is transmitted from one pair to the other, a novel apparatus isobtained that has two stable states and is adapted to be shifted fromeither one of its stable states to the other by application of aradiation trigger signal to the respective photoconductor coupled to thedark electroluminescent cell. Such a bistable apparatus is extremelyuseful, especially in digital computers for register and counterelements. When used as a register element, the two stable states wouldbe designated respectively as the binary digits 1 and 0. Input radiationtriggering means is provided to independently switch the deviceto eitherof its stable states. When used as. a counter element, the device isswitched from the stable state in which it is operating to its otherstable state upon application of a common input trigger signal; that is,the device would return to a particular stable state after each twoinput trigger signals.

A bistable device employing these electroluminescent phosphors has theadditional desirable feature of providing a visible indication of thestate of the device. Thus, one electroluminescent phosphor glows onlywhen the network is in one of its two stable states and the otherelectroluminescent phosphor glows only when the network is in the otherstable state.

It is therefore a principal object of this invention to provide abistable device comprising two mutually interrelated electro-opticalpairs.

Another object of this .invention is to provide a novel bistableelectro-optical network.

Another object of this invention is to provide an elect-rical networkincluding electroluminescent phosphors and photoconductors as elementsthereof and adapted to operate in either one of two stable states.

Another object of this invention is to produce an output signal from anelectro-optical network in response to every two input signals.

Another object of this invention is to provide an electrical networkincluding electroluminescent phosphors and photoconductors as elementsthereof and adapted to operate in either one of two stable states,wherein the particular phosphors luminescing are indicative of thestable state in which the network is operating.

The foregoing objects are achieved by providing networks having firstand second electro-optical pairs connected in parallel. A source ofelectrical energy is coupled across the parallel-connectedelectric-optical pairs. A point of the first electro-optical pairbetween the electroluminescent cell and the photoconductor thereof iscoupled to a point of the second electro-optical pair between theelectroluminescent cell and the photoconductor thereof. This couplingbetween the first and second electro-optical pairs is one means wherebyfeedback is provided so that the network can operate in only one of twostable states; that is, wherein the on electrooptical pair drawsrelatively large current and its electroluminescent phosphor emits arelatively intense radiant energy signal, and the off electro-opticalpair draws relatively little current and its electroluminescent phosphoremits relatively little, if any, radiant energy. Upon application of aradiant energy trigger signal to the photoconductor of the olf"electro-optical pair the network shifts to its other stable state. Inthis other stable state the formerly off electro-optical pair is on andvice versa. a

The invention will be described with reference to th accompanyingdrawings, wherein:

FIGURE 1 is a circuit diagram of the bistable network of this invention,including one form of triggering means;

FIGURE 2 is a perspective view, partly in section, of an electro-opticalpair useful in the circuit of FIG. 1.

In the bistable network of FIG. 1 an electro-optical pair 62 isconnected in parallel with an electro-optical pair 63. Pair 62 comprisesa series-connected electroluminescent cell 66 and photoconductor 65positioned in radiation-coupled relationship indicated by the arrow andbroken line between them. The arrows and broken lines elsewhere in FIG.1 indicate the same radiationcoupled relationship and this conventionwill be employed throughout this application. Electro-optical pair 63comprises'a series-connected electroluminescent cell 68 andphotoconductor 67 positioned in radiation-coupled relationship.Appropriate feedback, to be described hereinafter, causes this networkto operate in only one of of two stable states. 1

Examples of electro-optical pairs useful in the circuit of FIG. 1 areshown and described in the aforementioned U.S. patent application SerialNo. 585,052 by C. P. Spitzer. A device 31 including such anelectro-optical pair is shown in FIG. 2. Device 31 is adapted to receiveeither or both electrical and light input-signals and to produce eitheror both electrical and light output signals. In device 31 an electrode32 consists of a rigid opaque metallic member serving as both anelectrode and a supporting member and which is preferably polished formaximum light reflection. Deposited on one side of electrode 32 are anelectroluminescent layer 33, a light-transmitting electrode 34, aphotoconductive layer 35, and a light-transmitting electrode 36.Deposited on the other side of electrode 32 is an electroluminescentlayer 43, a light-transmitting electrode 44, a photoconductive layer 46and a light-transmitting electrode 47. Lead wires are soldered orotherwise electrically connected to each electrode.

Electroluminescent layer 33 and adjacent electrodes 32 and 34 comprisean electroluminescent cell EL-l, and electroluminescent layer 43 andadjacent electrodes 32 and 44 comprise another'electroluminescent cellEL-2. The above two electroluminescent cells are connected in series bythe common electrode-32 so as to make the cells EL-l and EL-2effectively one electroluminescent cell which is in radiation-coupledrelation with two photoconductors for reasons that will appear later.Similarly, photoconductive layer 35 and its electrodes 34 and 36comprise one photoconductor PC-l and the photoconductive layer 46 andits electrodes 44 and 47 comprise a second photoconductor PC-2. A casing48 which consists of the light-opaque, electrically-insulating materialis used to, support the cells, photoconductors and a lens 49 adjacentelectrode 36.

It should be understood that the word light, as used in thisapplication, includesany radiation emitted by an electroluminescentphosphor to which a photoconductor is responsive and may for example,include ultraviolet or infrared radiation. 7

Light-transmitting, electrical conducting electrodes 34 and 36 may belayers of titanium dioxide or tin oxide, commonly referred to asconducting glass. Alternatively, a very thin light-transmitting layer ofevaported metal, such as aluminum or silver, may be used. If thelighttransmitting, electrical conducting electrodes are titanium dioxidethey may be prepared and rendered conductive in accordance with theteachings of U.S. Patent No. 2,717,844 to L. R. Koller.

Electroluminescent layers 33 and 43 may be phosphors such as zincsulfide activated by three-tenths percent by weight of copper andwritten as ZnS:Cu, prepared as a continuous crystalline layer, asdisclosed in the abovementioned patent 2,709,765 to L. R. Koller, orsingle crystalline phosphors of the type disclosed in the abovementionedPatent 2,721,950 to Piper and Johnson. These types of electroluminescentlayers are responsive to both direct or alternating electric fields. Theaverage brightness B of the light output of an electroluminescentphosphor as a function of the voltage V applied to it may be closelyapproximated by the expression,

troluminescent phosphor used and k is a constant of proportionality.Values of n, in Equation 1, range approximately from 1 to 7 for knownphosphors.

Photoconductive layers 35 and 46 are thin light permeable layers ofphotoconductive material. The material may, for example, comprisecadmium sulfide or lead sulfide, which may besprayed, sputtered, orevaporated on one of the light-transniitting electrodes 34 or 36 and 44or 47. More generally, photoconductive layers 35 and 46 may, forexample, consist of any of the sulfides, selenides, or tellurides ofcadmium, lead, or zinc, or may be any other known photoconductor.

The physical arrangement of the device 31 is such that light emitted byelectroluminescent cell EL-l falls on photoconductor PC-l. This cell andphotoconductor are connected in electrical series relation betweenterminals 40 and 42, and therefore constitute an electro-optical pairsuch as pair 62 or 63 (FIG. 1). The current drawn by this pair dependson the voltage applied to the terminals light transmitting electrodes 34and 44. Each of electroluminescent cells EL-l and EL2, isradiation-coupled to its respective photoconductor indicated generallyby PC1 and F02. The relationship between the diagrammatic illustrationof FIG. 1, for example, and the physical embodiment of FIG. 2 may beunderstood, by noting that there is one device 31 for each of theelectro-optical pairs 62 and 63 (FIG. 1) and their radiation-coupledphoto conductors 72 and 71, respectively.

Referring once more to FIG. 1, electro-optical pairs 62 and 63 areconnected in parallel across a source of electrical energy, such asconstant current source 64. Electro-optical pair 62 comprises aphotoconductor 65 and an electroluminescent cell 66 connected in series.Electro-optical pair 63 comprises a photoconductor 67 and anelectroluminescent cell.68 connected in series. A connection point 69 isprovided between photoconductor 65 and electroluminescent cell 66 and aconnection point 70 between photoconductor 67 and electroluminescentcell 68. A pair of series-connected photoconductors'71 and 72 isconnected between points 69 and 7t Photoconductors 71 and 72 arepositioned in radiation-coupled relationship with respectiveelectroluminescent cells 68 and 66. The network is switched from onestable state to another upon application of a radiation input triggersignal from atrigger source 74 to a photoconductor 73 connected betweenthe photoconductor ends of electro-optical pairs 62 and 63 and thecommon connection point of the series-connected photoconductors 71 and72.

In the operation of the circuit of FIG. 1, current I from source 64substantially divides between electro-optical pairs 62 and 63. Thenetwork is in its designated first stable state when electro-opticalpair 62 is on and in its designated second stable state whenelectro-optical pair 63 is on.

Assume now that the circuit is in its first stable state.Electroluminescent cell 66 is lighted and illuminates photoconductors 65and 72. Electroluminescent cell 68 is dark and consequentlyhotoconductors 67 and 71 are not illuminated and have a high resistance.The greater portion of current I therefore flows through electro-opticalpair 62. Upon application of a rediation input trigger signal fromtrigger source 74 to photoconductor 73, a relatively low resistance isprovided for current flow through photoconductors 73 and 72 toelectroluminescent cell 68. The impedance of photoconductors 72 and 73in parallel with the decreasing impedance of photoconductor 67 providean impedance which is low as compared with that of photoconductor 65.The light output of elec troluminescent cell 68 thereupon tends toincrease and the resistance of radiation-coupled photoconductor 67 tendsto decrease so that a greater portion of current I is drawn throughelectro-optical pair 63. The increase of current through electro-opticalpair 63 is accompanied by a decrease in current through electro-opticalpair 62, a consequent decrease in the light output of electroluminescentcell 66, and a consequent increase in the resistance of photoconductor65. The current in electro-optical pair 63 continues to increase andthat in electro-optical pair 62 continues to decrease while theradiation input trigger signal is applied to photoconductor 73, untilelectro-optical pair 63 has reached its stable on condition andelectrooptical pair 62 has reached its stable off condition. At thistime the circuit is in its second stable state. Upon application of thenext radiation input trigger signal to photoconductor 73, passage ofcurrent through photoconductor 71, which has been illuminated byelectroluminescent cell 68, serves to change the state of the networkonce again. Thus, the network of FIG. 1 is changed from one stable stateto the other by application of an electrical signal to the offelectro-optical pair.

Photoconductors 71 and 72 must have response times relatively slowcompared to the time necessary for the network to switch from one stablestate to the other in order that their resistances remain either high orlow sufiiciently long for the switching operation to reach completion.

A resistor 75 is included in series relationship with electro-opticalpair 63 and serves to effectuate resetting or initial setting of thebistable circuit upon the respective removal and reapplication orinitial application of the source of current. With the source of current64 removed, both electro-optical pairs 62 and 63 are off. When currentsource 64 is applied to the network, current increases more rapidly inelectro-optical pair 62 than in electrooptical pair 62. Thus the networkwill assume its first stable state.

In the above description of the operation of the circuit of FIG. 1 itmay be seen that electro-optical pairs 62 and 63 connected in parallelto constant current source 64 constituted a bistable network. Feedbackis provided by the high internal impedance of constant current source64. It is within the scope of this invention to provide other means forelectrically triggering this bistable network than that shown in FIG. 1.Furthermore, this triggering means may be applied so that the networkoperates as either a register element or as a counter element.

A theory of the operation of the bistable apparatus of this invention,as presently understood, and a method of determining certain circuitparameters are described in a copending application Serial No. 705,680,filed December 27, 1957 now Patent No. 2,997,596, August 22, 1961, fromwhich the present application has been divided.

While the principles of the invention have now been made clear inillustrative embodiments, there will be immediately obvious to thoseskilled in the art many modifications in structure, arrangement,proportions, the elements, materials, and components, used in thepractice of the invention, and otherwise, which are particularly adaptedfor specific environments and operating requirements, without departingfrom those principles. The appended claims are therefore intended tocover and embrace any such modifications, within the limits only of thetrue spirit and scope of the invention.

What is claimed is:

1. A signal responsive network comprising a pair of electrical circuitbranches, each of said branches comprising an electroluminescent celland a photoconductor connected in series, one end of each of saidbranches being connected to a first common point, the other end of eachof said branches being connected to a second common point, theelectroluminescent cell and the photoconductor of one of said branchesbeing positioned in radiationcoupled relationship, and theelectroluminescent cell and the photoconductor of the other of saidbranches being positioned in radiation-coupled relationship, a resistorconnected in series in one of said pair of branches, a feedback meanscoupled between the first and second electro-optical pairs for providingfeedback which causes the network to operate in only one of two stablestates when a constant current source is connected in parallel with saidpair of electrical circuit branches, and means for connecting a constantcurrent source in parallel with said pair of electrical circuitbranches.

2. A counter network element comprising at least five electricallydistinct connection points, a first photoconductor connected between thefirst and second of said points, a first electroluminescent cellconnected between the second and third of said points, a secondphotoconductor connected between the first and fourth of said points, asecond electroluminescent cell connected between the third and fourth ofsaid points, the first electroluminescent cell and the firstphotoconductor being further positioned in radiation-coupledrelationship, the second electroluminescent cell and the secondphotoconductor being further positioned in radiation-coupledrelationship, a third photoconductor connected between the second andfifth of said points, a fourth photoconductor connected between thefourth and fifth of said points, the first electroluminescent cell beingfurther positioned in radiation-coupled relationship with the fourthphotocon ductor, the second electroluminescent cell being furtherpositioned in radiation-coupled relationship with the thirdphotoconductor, and a fifth photoconductor connected between the firstand fifth of said points.

3. A network element as defined in claim 2 further including a constantcurrent source, and means for connecting said source to said first andthird points.

4. A network element as defined in claim 3 further including triggeringmeans positioned in radiation-coupled relationship with said fifthphotoconductor and adapted to apply a radiation input trigger signalthereto.

5. A binary counter network comprising a first electroluminescent celland a first photoconductor electrically connected in series, said firstelectroluminescent cell being positioned in radiation-coupledrelationship with said first photoconductor, a second electroluminescentcell and a second photoconductor electrically connected in series, saidsecond electroluminescent cell being positioned in radiation-coupledrelationship with said second photoconductor, a third photoconductorpositioned in radiationcoupled relationship with said secondelectroluminescent cell and having one terminal connected to a junctionbetween said first electroluminescent cell and said firstphotoconductor, a fourth photoconductor positioned in radiation-coupledrelationship with said first electroluminescent cell and having oneterminal connected to a junction between said second electroluminescentcell and said second photoconductor, means for connecting a constantcurrent source in parallel with the two series circuits consisting ofsaid first electroluminescent cell in series with said firstphotoconductorand said second electroluminesconductor and triggeringmeans positioned in radiation cent cell in series with said secondphotoconductor, and coupled relationship with said fifth photoconductorfor switching means connected to a second terminal of said applying aradiation input trigger signal thereto.

third photoconductor and to a second terminal of said fourthphotoconductor for momentarily coupling said 5 B third and fourthphotoconductors in parallel with said References Cited In the file ofthls Damnt first and second photoconductors, respectively. UNITED STATESPATENTS 6. A binary counter network as defined in claim 5 wherein saidswitching means consists of a fifth photo- 2,895,054 Loebner July 14,1959

